1. Field of the Invention
The invention relates to a blow box for suspended guidance and/or conveyance of strip material or sheets. More particularly, the invention is concerned with blow boxes in which the blow means is blown with the effect of the blow jet issuing from a slot nozzle at an incline against the strips or sheets to be guided and/or conveyed.
2. Description of the Prior Art
Blow boxes with slot nozzles are known in various designs. In one known design, the blow box is produced in cast aluminum. In this manner, slot widths can be produced with a high degree of accuracy. However, the high production costs for these cast aluminum boxes are a disadvantage. Therefore, to avoid high production costs the blow boxes are in most cases produced from sheet metal. With the sheet metal blow boxes, it is difficult to maintain a constant slot width over the entire length of the slot. This is difficult even when the nozzle lips are held at a distance by means of bars inserted in the slot. However, not only production tolerances, but also deformations caused in the nozzle lip in operation by the generally hot blow means make it practically impossible to maintain the slot width within narrow tolerances.
There are blow boxes of other types which have blow openings and which aim at producing an area effect such as those disclosed in German Auslegeschrift No. 1,907,083; but, these also are not pertinent to the teachings of the invention, as they do not produce a slot jet, as will be further discussed subsequently.
Narrow tolerances of the slot width are necessary, however, to achieve optimum suspension characteristics. If the strip or sheet, for instance, is guided at a distance of one milimeter from the barrier wall facing the strip or sheet, then according to experience, the accuracy required with a slot width of one millimeter amounts of 0.2 mm. Only when it is possible to guide the strip material absolutely free of contact with the blow box within such a narrow space, can it be guaranteed that strips of material such as paper cannot become deformed in a suspension dryer fitted with such blow boxes due to strain and moisture during drying. Experience with offset dryers, which do not guarantee such a narrow guidance of the strips, has shown that longitudinal waves of about 1 mm high form during drying. These longitudinal waves cause loss in quality of the strips. Attempts have already been made to solve this problem by drying the strip material with hot steam or a combination of hot steam and hot air instead of with dry air. This way of drying has the disadvantage that it comsumes a large amount of energy.
The object of the invention is to produce a blow box for suspended guidance and/or conveyance of strip material or sheets which has the properties of a blow box with slot nozzles and can be produced with a small slot width with a very high degree of accuracy and lower production costs than in previous cases. This object is solved according to the invention with a blow box of the aforementioned type in that blast openings are arranged in the barrier wall facing the strips or sheets so as to be close to one another in a row and have blowing directions which are substantially the same or which slightly diverge towards the ends of the blow box. Each blast opening is formed by a tongue and an inclined guide surface having edges which overlap into the barrier wall. The tongue is positioned in the barrier wall, and the tip of the tongue is sunk or extend inwardly into the blow box. The inclined guide surface is also sunk or extended inwardly into the blow box, and the edges of the guide surface which overlap into the barrier wall and the guide surface are arranged with its edges around the tongue. Such a blow box according to the invention can be produced with the precision of the blow box which is in cast aluminum, and the blow box according to the invention can be produced at a considerably lower cost. The blow box preferably produced by means of a punched-out portion which is cut with a stamp or die and punch. Whereas in the previous blow boxes provided with slot nozzles, several sheet metal parts had to be individually composed to form the blow box, and the spacing elements had to be distributed in the slot to maintain a definite slot width over the length of the slot. The blow box according to the invention is produced from a piece of sheet metal in a single working step.
The jets issuing divergently out of the individual blast openings, which are arranged closely together in a row and form short arc-shaped slots, join to form a solid blow jet and then have the effect of a blow jet issuing from a continuous slot. As the slot blow openings are only short in length, their width can be produced with very high precision. For the same reason, no deformations which change the width of the slot occur in operation when the blow box is heated by the hot blow means. Thus, the basic pre-conditions for a narrow guidance of the strips are achieved. The front edge of strip material is very difficult to guide without contact even with wider distances, and in order to assure that the front edge of the strip material does not come into contact with the tip of the tongue even in the central area of the sunken contact surface, the tip of the tongue is also sunk into the blow box. Due to the structure of the guide surface and its lateral edges, which overlap into the barrier wall, on the one hand and the tongue with its lowered tip on the other, the width of the slot can be made substantially constant over its entire length and with it also the blow jet over its entire width.
Similar blow openings are also known in blow boxes of other types mentioned earlier (German Auslegeschrift No. 1,907,083), but these known blow openings are intended to produce an area effect and not the effect of a slot jet. The blast openings are therefore not arranged in a row in the barrier wall, but are distributed over the entire barrier wall. In contrast to the aforementioned prior known blow box, the blast openings of the blow box according to the invention are constructed so that the tongues are positioned in the surface of the barrier wall. The guide surface is structured in the form of a circular sector and has substantially radially running edges. The angle of opening of the blast openings is between 120.degree. and 180.degree. so that it is more of a broad-fanned swell flow than a jet with a defined direction which issues out of the blast openings. Due to the described features, such blast openings cannot be arranged in a row sufficiently close together positioned to guide a strip spaced at a very narrow distance (e.g. 1 mm) from the barrier wall with the effect of a slot jet so as to be safely without contact.
Optimum suspension properties (in contrast with the aforementioned known blow box) are achieved with a blow box according to the invention in which the lateral edges of the guide surface run substantially parallel. In such a case, the blow jets issue only slightly divergent from the arc-shaped slots so that the adjacent edge jets of the blow jets hardly disturb each other, and the kinetic energy of the blow jets practically completely comes to bear on the strip material; however, the divergence is still sufficient to form a solid slot jet. The blow box according to the invention can be structured in several ways. In one embodiment of the invention, the barrier wall is provided with the blast openings of the type heretofore described according to the teachings of the invention in a pair of rows to direct the blow jets in a direction towards each other.
According to another embodiment, the barrier wall has a perforation or openings for the outlet of additional blow means in front of the row or rows of blast openings. The additional blow means issuing through this perforation serves to prevent the strips or sheets from touching the barrier wall, even in the area of the blow box where no blast openings are provided and the effect of the blow means out of the blast openings is no longer present. According to a further embodiment of the invention, two rows of blast openings are arranged with one row behind the other row and with both rows directing the jet in substantially the same blowing direction. The blast openings of the second row are arranged in staggered relationship to the blast openings of the first row.
In the embodiments with two rows of blast openings lying opposite one another, however, the blowing directions of these openings are preferably directed towards one another.
A blow box particularly suited for stretching the strip or sheet is characterized in that in the case of two rows of blast openings lying opposite one another, the blowing directions of these openings are directed away from one another and the barrier wall has a perforation between the rows for the outlet of additional blow means. In such a case, the blast openings of each row preferably diverge from the center of the blow box towards the end.
As already mentioned, such blow boxes are used in suspension dryers. Blow boxes with rows of blast openings lying opposite one another are above all suited for suspension dryers. The arrangement of the blow boxes in a suspension dryer is preferably above or below the strip material or sheets to be guided and/or conveyed and the upper blow boxes are arranged in staggered relationship with respect to the lower blow boxes. In contrast to the dryers operated with hot steam or with hot steam and hot air combined, in such a dryer, the drying process itself is not affected but longitudinal waves are prevented from forming by guiding the strips at a short distance (approximately 1 mm) from the barrier walls of the blow boxes. Namely, the blow boxes have a pressing effect on the strip material with this short distance.
Other objects, advantages and the nature of the invention will become readily apparent from the detailed description of the preferred embodiments as described in connection with the accompanying drawings.